Multilayer thick film circuits have been used for many years to increase circuit functionality per unit of area. Moreover, recent advances in circuit technology have placed new demands on dielectric materials for this use. Heretofore, most of the dielectric materials used in multiple circuits have been conventional thick film dielectric compositions. These are comprised of finely divided particles of dielectric solids and inorganic binders dispersed in an inert organic medium. Such thick film materials are usually applied by screen printing, though they may be applied by other means as well. Thick film materials of this type are very important and will continue to be so.
In constructing a multilayer circuit using thick film materials, it is necessary sequentially to print, dry and fire each functional layer before the next layer is applied. Thus, in a typical situation involving multicircuits having, say, twenty layers, sixty separate processing steps are required as well as twenty inspections to assure the quality of each of the processed layers. Such a complex process is, of course, expensive both because of the great number of steps and because of the high yield losses which are normally incident to such a complex procedure.
Another approach to this problem has been the use of dielectric tapes which are thin sheet of ceramic dielectric material, such as alumina. The tape process involves lamination of a number of sheets of unfired tape (usually alumina) interspersed with alternating printed layers of conductive material. However, very high temperatures (e.g. on the order of 1600.degree. C.) are required to sinter the alumina. Thus, only very high melting conductive materials such as tungsten and molybdenum can be used. Unfortunately, molybdenum and tungsten have poor conductivity properties which make them less satisfactory for very high speed, highly complex circuitry.
Recently, low temperature co-fired (LCTF) technology has been introduced as a method for fabricating multilayer circuits. This technology offers the combination of the processing advantages of the alumina-based tape technology plus the materials advantages of thick film technology. The LTCF tape systems have firing temperatures below 1000.degree. C. and allow the use of high conductivity metals such as silver, gold, platinum, nickel and copper.
However, these tapes do have certain disadvantages. Most dielectric tape systems have problems with excessive movement of conductor patterns when the parts are fired more than once. In general, the multilayer structure is laminated together, fired and then a surface metallization is applied and the part is fired again. During this second firing step, the fluidity of the ceramic material may allow shifting or distortion of the conductor patterns. This, in turn, prevents attainment of the rigid tolerances for dimensions that have to be met. In addition, many of the tape systems have dielectric constants (K) between 6 and 8 which result in unacceptably high propagation delays in multilayer interconnect systems. Some tape systems suffer from high dielectric loss. And some tape systems have thermal coefficients of expansion (TCE) which do not match the TCE's of components or other substrates.
From the foregoing, it can be seen that there is a substantial need for a low temperature co-fireable tape dielectric which (1) retains dimensional stability even during multiple firing steps; (2) has a low dielectric constant (less than 5); (3) has low dielectric loss; and (4) has a variable TCE so that the TCE can be matched to the components or to other substrates.